The invention relates to a sensor and, more particularly, to a sensor provided with a thin membrane, capable of storing electrical charges, whose surface is in contact with the chemical, biological, and/or other physical parameters to be measured. The membrane is part of an electromechanical resonance circuit, operating with propagating acoustic waves. Provision is made opposite the membrane for converters, equipped with interdigital structures, to act as transmitter and receiver for the propagating acoustic plate waves. Each of the converters is equipped with interdigital structures that act as narrow band filters. The transmitter converter and the receiver converter is arranged with a distance between them. The signal from the receiver converter is detected capacitively. A suitable electronic control and evaluation circuit or feedback is provided in addition between the output signal of the receiver converter and the transmitter converter. Finally, a DC voltage is applied between the membrane on the one hand and the transmitter converter and the receiver converter on the other. The voltage has an alternating voltage superimposed on it in the transmitter converter.
Technical advancement has made it necessary to detect the presence of chemical or biochemical substances and to determine their concentrations, and to measure other physical parameters and their changes in the form of electrical signals. Sensors are used for this application. The sensor's theoretical purpose consists in the conversion of chemical and biochemical reactions, or the detection of the existence of or change in other physical parameters, into electrical signals, frequently accomplished with electronic means. The areas of application for such sensors include, for example, process monitoring, detection of impurities, performance of analyses, exhaust emissions monitoring, waterways monitoring, gas alarm systems, and medical technology, etc.
Mass-sensitive converters, also called gravimetric sensors, have been discussed for use as high-sensitivity sensors. This group of sensors reacts, for example, to the accumulation of, or a change in the accumulation of, the desired substance on the sensor on which a chemically active coating is provided for this purpose. The sensors are designed as an electromechanical resonance circuit. By using electrical oscillations and suitable feedback, the mechanical components of the sensor are set oscillating. If continuous waves are used in the resonator, the resonant frequency is determined by the phase velocity of the waves and possibly by the impressed wavelength.
Thus, for example, sensors are known that work with acoustic surface waves. A piezoelectric layer is used as the mechanical part of the resonator. Interdigital structures are evaporated onto the piezoelectric layer. The structures then, generate continuous acoustic surface waves under electric excitation that are emitted along the piezoelectric layer. Another interdigital structure applied to the piezoelectric layer acts as a receiver, receiving the acoustic surface waves emitted by the transmitter and converting them into an electrical signal. An electronic amplifier is provided between the transmitter and the receiver. The amplifier provides feedback between the receiver and the transmitter as well as compensation for losses. This produces an oscillating system that is free of damping.
The mechanical oscillator can be provided on one or both sides with a chemically active coating that is selected for the desired substance. The desired substance binds on the chemical coating of the substrate. This causes a change in mass or a change in the surface properties of the substrate. This in turn leads to a change in the resonant frequency or a change in the propagation rate of the waves. This causes a frequency shift in the resonant circuit, so that the deposition of the desired substance can be evaluated by an electrical signal.
The propagation rate of the acoustic surface waves is first determined by the material properties of the piezoelectric layer and, second, by their surface properties. Changing the properties of the oscillator changes the oscillator frequency. The sensitivity is inversely proportional to the wavelength and directly proportional to the frequency. Acoustic surface wave sensors can be operated at high frequencies of several hundreds of MHz. This gives them high sensitivity. Operating at high frequencies has disadvantages, however. Expensive, cumbersome, and trouble-prone electronics and a design using high-frequency criteria are required. A sensor of this kind radiates VHF waves, and the short wavelengths require very fine interdigital structures that pose manufacturing problems.
An important aspect of using acoustic surface wave sensors is the high propagation rate of the surface waves. Normally, it is far above the speed of sound in water or in other fluids. This results in a failure of the sensors when they are used in fluids. If the propagation rate of an acoustic surface wave is above the speed of sound of an ambient fluid, the wave dissipates in the fluid since the energy loss results in a sharp damping of the surface wave.
The use of surface wave sensors is therefore limited as a rule to gas or vapor detection.
Another class of gravimetric sensors is composed of sensors that work with acoustic plate waves. These acoustic plate waves propagate on a thin membrane and have much lower propagation rates at the same wavelength than those in sensors that use acoustic surface waves. The typical design of a sensor that uses acoustic plate waves relies on the structure of a resonant circuit with acoustic surface waves. A transmitter converter provided with interdigital structures and a receiver converter provided with interdigital structures are combined by means of an electronic amplifier with feedback and a thin membrane into an electromechanical resonator. The transmitter converter generates acoustic traveling plate waves that are picked up by the receiver converter. The Electromechanical conversion therefore involves a piezo effect. An aluminum layer is first evaporated onto a silicon nitride membrane; the aluminum layer then serves as a carrier for the piezoelectric layer made of zinc oxide, applied for example with electron-magnetron sputtering. Finally, the transmitter and receiver interdigital structures are evaporated onto the zinc oxide layer. The finger groups and finger numbers are designed so that one period (finger pair) corresponds exactly to a wavelength during operation at the resonant frequency. The wavelengths are between 10 and 1000 lm as a rule. This membrane serves as the mechanical part of the resonator and can be provided with a chemically active layer that selectively acts toward certain substances.
"Sputtering" of the zinc oxide layer produces compressive stress. In many cases, this causes waves in the membrane or breakage of the layer and similar problems. These problems increase with the thinness of the membrane. Therefore, it has not been previously possible to make any composite membrane thinner than about 3 lm. Since the sensitivity of the sensor increases inversely with the thickness of the membrane, the sensitivity of an acoustic plate wave sensor using the piezoelectric layer cannot be further increased.
Another research approach to a gravimetric sensor uses electrostrictive excitation of plate waves. This avoids "sputtering" of a piezoelectric layer. The structure is as follows:
A thicker aluminum layer is evaporated onto one side of a thin silicon nitride membrane. The interdigital structures of the transmitter converter and receiver converter are applied to the other side of the silicon nitride membrane. The acoustic plate waves are generated as follows: Between the fingers of the interdigital structures and the aluminum layer, a direct voltage is applied, superimposed on an alternating voltage to generate the acoustic plate waves. The electrostrictive effect is then in the form of a change in volume of the dielectric by polarization. The electric dipoles located sequentially in the direction of an outer electrical field exert an attractive force on one another in which the molecules approach one another until the elastic counterforces balance the electrical forces.
The mechanical stresses coupled in this fashion generate plate waves that propagate along the composite membrane. In the receiver converter, the advancing plate waves cause changes in thickness and in the dielectric constant of the nitride layer, resulting in turn in a change in the capacitance of the capacitors composed of an aluminum layer and fingers. This can be converted into an electrical signal. Electrical decoupling as a result of the change in thickness has merely been postulated but not demonstrated in practice.
Therefore, only relatively low amplitudes of the acoustic plate waves can be achieved by the electrostrictive method. The orders of magnitude of the idle voltage and the short-circuit current of the receiver converter are correspondingly small. Only a modest signal-to-noise ratio can be achieved.
Another problem with electrostrictive excitation methods is the high capacitance of the transmitter converter. Under the control of the typical exciting voltages, reactive powers in the range from 10 W appear, causing a large power loss in the control electronics and interfering with its miniaturization. Hence, the feasibility of a feedback resonator using this principle is very questionable.
Hence, there is needed a simple and economical acousto-gravimetric sensor suitable for mass production equipped with a membrane that is a part of an electromechanical resonator circuit, the circuit being suitable for measuring gaseous and liquid media, having a very high sensitivity, and also allowing the use of a membrane that is as thin as possible that also permits determination of the position of the sensor with high resolution.
These needs are met according to the present invention by a sensor provided with a thin membrane, capable of storing electrical charges, whose surface is in contact with the chemical, biological, and/or other physical parameters to be measured. The membrane is part of an electromechanical resonance circuit, operating with propagating acoustic waves. Provision is made opposite the membrane for converters, equipped with interdigital structures, to act as transmitter and receiver for the propagating acoustic plate waves. Each of the converters is equipped with interdigital structures that act as narrow band filters. The transmitter converter and the receiver converter is arranged with a distance between them. The signal from the receiver converter is detected capacitively. A suitable electronic control and evaluation circuit or feedback is provided in addition between the output signal of the receiver converter and the transmitter converter. Finally, a DC voltage is applied between the membrane on the one hand and the transmitter converter and the receiver converter on the other. The voltage has an alternating voltage superimposed on it in the transmitter converter. The generation of the acoustic plate waves in transmitter converter is produced solely on the basis of electrostatic attractive force or excitation between interdigital structures of the transmitter converter and the membrane. The output signal of the receiver converter is additionally decoupled in a capacitive manner in such fashion that the vibration amplitude of the acoustic propagating plate waves is measured in the receiver converter by the capacitor formed by interdigital structures of the receiver converter and the membrane. Finally, the membrane and interdigital structures of the transmitter converter on the transmitter side and the membrane and interdigital structures of the receiver converter on the receiver side are arranged spaced apart from one another by a spatial distance in the form of a gap. They have no mechanically solid connection with one another. A dielectric in the form of a narrow gap is therefore formed such that movement of the membrane toward the interdigital structures is possible in the dielectric.
The advantages of the present invention consist, in particular, in that the generation of acoustic propagating plate waves takes place at the transmitter solely by the electrostatic method, while at the receiver end the input and decoupling of the output signal are detected purely by capacitive means. This makes it possible for the dielectric between the membrane and the interdigital structures on the transmitter and receiver side to consist of air, so that undamped oscillating movements of the membrane are possible and a dielectric is chosen such that these oscillating movements of the membrane remain possible. The function of the sensor according to the present invention is ensured in both gaseous and liquid media.
Other advantages consist in the fact that by separating the interdigital structures of the transmitter converter and the receiver converter from the membrane, the acoustic plate waves can be generated on any membranes or foils. By separating these interdigital structures and the membrane, various converters can be produced. As a result, the possibility of developing position sensors is created. Extremely thin membranes can be used in the electromechanical resonant circuit, since when a pure metal membrane is used, for example, there is no problem of two-layer materials working against each other because of different coefficients of expansion. The very thin membranes also permit an extremely high sensitivity for the sensor according to the invention.
In the electrostatic method, the acoustic plate waves can reach amplitudes of about 800 .ANG.. In the receiver converter, the idle voltage is higher than in the electrostrictive method. This results in a much higher output voltage and a very high signal-to-noise ratio for the electrostatic method in contrast with the electrostrictive method. In terms of both the order of magnitude of the idle voltage and the short-circuit current, as well as the noise distance, the electrostatic method according to the invention is superior to the electrostrictive method. It offers far greater reserves for additional electronic processing, and capacitive parasitic effects and interference have much less of an effect because of the higher level.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.